1. Field of the Invention
The present invention relates to an improved process and apparatus for carrying out partially liquid phase reactions coupled with reactions in a catalytic distillation.
2. Related Art
It is well known that primary alcohols will react preferentially with the tertiary alkenes in the presence of an acid catalyst, for example, as taught in U.S. Pat. Nos. 3,121,124; 3,629,478; 3,634,534; 3,825,603; 3,846,088; 4,071,567; and 4,198,530.
U.S. Pat. Nos. 5,003,124 and 4,950,803disclose a liquid phase process for oligomerization of C4 and C5 isoolefins or the etherification thereof with C1 to C6 alcohols in a boiling point fixed bed reactor (boiling point reactor) that is controlled at a pressure to maintain the reaction mixture at its boiling point which may be directly attached to a catalytic distillation reactor. The two reactors are then operated under a single pressure as determined by the overhead of the catalytic distillation tower.
The advantage of combining the two types of reactor is that the partial liquid phase reactor acts as a guard bed to the more expensive catalytic distillation column and converts a portion of the reactants to products, thus requiring less of the catalytic distillation structure. The conversions are usually around 85-90% while the catalytic distillation gives conversions of 97+%.
The catalytic distillation process employs a catalyst system (See U.S. Pat. Nos. 4,215,011 and 4,302,356) which provides for both reaction and distillation concurrently in the same reactor, at least in part within the catalyst system. The method involved is briefly described as one where concurrent reaction and distillation occur in a combination reactor-distillation structure as described in several U.S. Patents, namely U.S. Pat. Nos. 4,242,530; 4,250,052; 4,232,177; 4,302,356; 4,307,254; and 4,336,407.
For example, in this system and procedure, methanol and isobutene containing C4 stream are continuously fed to the reactor/distillation column where they are contacted in the catalytic distillation structure. The methanol preferentially reacts with isobutene forming MTBE which is heavier than the C4 components of the feed and the methanol, hence it drops in the column to form the bottoms. Concurrently, the unreacted C4""s (e.g. n-butane, n-butenes) are lighter and form an overhead.
It is an advantage of the present system that the two reactors may be operated at different pressures. This and other advantages will become apparent from the following descriptions.
The present invention is an improvement that relates to a decoupling drum located between a boiling point fixed bed reactor and a catalytic distillation column. The decoupling drum allows a boiling point fixed bed reactor and a catalytic distillation column reactor to run at different operating pressures, thus allowing independent pressure control. The independent pressure control allows each reactor to be operated at their individual optimum conditions in order to maximize efficiency.
In one aspect the present invention is an improvement in the exothermic, partially liquid phase reaction of C4 and C5 isoolefins with themselves to form oligomers, preferably dimers, and with C1 to C6 alcohol to form ethers by contact in a boiling point reactor containing a fixed bed catalyst of acidic cation exchange resin, at a pressure to maintain the reaction mixture at its boiling point within the range of 120 degrees F. to 300 degrees F. whereby at least a portion but less than all of said reaction mixture is in the vapor phase said first reactor connected to a catalytic distillation column reactor having a fixed bed of catalyst, which also serves a distillation structure, wherein the improvement is the addition of a decoupling drum between said boiling fixed bed catalytic reactor and said catalytic distillation column whereby each reactor may be operated independently at different pressures.
The boiling point reactor is a substantial departure from the prior art for this type of reactor, where sufficient pressure was employed to maintain the entire reaction mixture in liquid phase.
A given composition, the reaction mixture, will have a different boiling point at different pressures, hence the temperature in the reactor is controlled by adjusting the pressure to the desired temperature within the recited range. The boiling point of the reaction mixture thus is the temperature of the exothermic heat of reaction which is dissipated by vaporization of the reaction mixture. For C4 feeds where isobutene is  greater than 25 percent, a recycle is also provided so as to control the vaporization and temperature in the range desired for the process. The maximum temperature of any heated liquid composition will be the boiling point of the composition at a given pressure, with additional heat merely causing more boil up. The same principle operates in the present invention to control the temperature. There must be liquid present, however, to provide the boil up, otherwise the temperature in the reactor will continue to rise until the catalyst is damaged. In order to avoid exotherms which will vaporize all of the reaction mixture, as stated earlier, a recycle is necessary to limit the amount of vaporization and reactor temperature for the process. In this mode the process virtually can handle any concentration of isoolefins in the feed.
In another aspect the present invention is an improvement to a coupled liquid phase fixed bed boiling point reactor and a catalytic distillation column, wherein the improvement comprises a decoupling drum located therebetween. The decoupling drum allows the fixed bed boiling point reactor and the catalytic distillation column to run at different operating pressures, thus allowing independent pressure control. The independent pressure control allows the reactor and catalytic distillation column to individually operate at their optimum conditions in order to maximize efficiency and reduce the recycle requirements as some of the heat of reaction goes into vaporization. Other embodiments may include two fixed bed boiling point reactors in series, with the exit effluent from the second reactor connected to a decoupling drum before going to the catalytic distillation column reactor. The addition of the decoupler drum will allow the first boiling point reactor to operate at lower temperatures and pressures than when directly connected to the catalytic distillation column reactor. The lower pressure in the first boiling point reactor, results in lower temperature operation with more favorable conversion and selectivity. Any need for recycling the reactor effluent in the first boiling point reactor is reduced due to the increased efficiency. In addition, the increased efficiency allows for a smaller size boiling point reactor resulting in lower equipment and operating costs.
The present reaction scheme is the preferred operation on streams containing large amounts of the isoolefin, however, feed to the reaction will need to be preheated. Since high concentrations of isoolefin (25 to 60 wt. % for C4 feed) provide a great exotherm, hence liquid recycle is used as a diluent to control the temperature in the reactor so as to provide limited vaporization. In any event it may be necessary to preheat the feed to the reaction such that temperature of the reaction, i.e., the boiling point of the reaction mixture (feed temperature plus exotherm) is in the range of 120xc2x0 F. to 300xc2x0 F., which represents the desirable range for the equilibrium reactions at a pressure in the range of 0 to 400 psig.
The catalyst bed in the liquid phase boiling point reactor may be described as a fixed continuous bed, that is, the catalyst is loaded into the reactor in its particulated form to fill the reactor or reaction zone, although there may be one or more such continuous beds in a reactor, separated by spaces devoid of catalyst. The catalyst bed in the distillation reaction column reactor is in such a form as to act as both the catalyst for the reaction and distillation structure for the fractional distillation.